# Questions tagged [groupoids]

A groupoid is a small category in which every morphism is an isomorphism. They arise throughout mathematics, e.g. in guise of fundamental groupoids in the theory of covering spaces, holonomy groupoids in the theory of foliations or Lie groupoids in mathematical physics. For groupoids in the sense of universal algebra, i.e., a set with a binary operation, please use the (magma) tag. Also, use other tags such as monoid or category-theory, if needed.

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### Classical Galois Theory and Groupoids?

Given your favorite field $k$, we can build its groupoid of algebraic closures in the obvious way: Look at the category of all possible algebraic closures with field isomorphisms as the arrows. This ...
2answers
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### Good book for self-study of Magmas/Semigroups/etc.?

I'm currently an undergrad in my second semester of Abstract Algebra. We've covered groups, rings, fields, all that fun stuff. I'm working with Shahriari's "Algebra in Action" as well as ...
1answer
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### Explicit description of pullback of $(2,1)$-categories.

Let us consider the ordinary category of $(2,1)$-categories. Its objects are groupoid enriched categories, and its morphisms are 2-functors. Is there an explicit way to define the objects, morphisms ...
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### Pushout of groupoid

I'm learning category theory. There's a homework question asking for a pushout of groupoid. Suppose $C_0,C_1,C_2$ are groupoids and denote $f_i:C_0\to C_i\quad i=1,2$ the functor. I have managed to ...
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### Hopf “algebroid” structure of a groupoid convolution algebra?

To male thinks simple as possible, lets say we have a discrete group $G.$ Then the then the group algebra $\mathbb{C}[G]$ (of finitely supported complex valued functions on $G$) has a convolution and ...
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### adjunctions from either the category of small categories or the category of groupoids

I'm looking for examples of adjunctions involving either the category Gpd of groupoids or the category Cat of small categories. The insertion of Gpd into Cat has both a left and a right adjoint, so ...
0answers
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### Reference request: Integration of A-paths.

Recall that an A-path for a Lie Algebroid is an an algebroid morphism from the tangent bundle over the unit interval $[0,1]= I$ to a general Lie algebroid $A$. Now, whenever A is the algebroid of some ...
2answers
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### Equivalence Between Category of Covering Spaces and Category of Sets with an Action By The Fundamental Groupoid

Let $\Pi_1(X)$ be the fundamental groupoid of a locally path-connected topological space $X$ and define $\Pi_1(X)-\mathbf{Sets}$ to be the category of sets equipped with an action by the fundamental ...
1answer
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### Functor From Category of Covering Spaces to Category of Sets Equipped With An Action By The Fundamental Groupoid

I have some problems with the understanding of the connection between covering spaces and the fundamental groupoid. Let $X$ be a topological space and let $\Pi_1(X)$ denote the fundamental groupoid of ...
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### classifying space induces a equivalence of categories between PBun$_G(M)$ and $\Pi(M,BG)$ for finite groups $G$

Let $G$ be a finite group, $BG$ its classifying space and M a manifold. Then it is mentioned in https://arxiv.org/abs/1705.05171 (Remark 2.3 d) that there is an equivalence of categories  \Pi (M,BG) ...
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### Notation for the scope of definition of a partial binary operation of a groupoid

I have a groupoid $(G,\ast )$ with a partial binary operation $*:G\times G\rightharpoonup G$. For every $(a,b)\displaystyle ∈G\times G$, $\displaystyle *$ is defined if ...
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### Convolution algebra of a groupoid

In this paper in order to explain conormal distribution the authors use the fact that “the elements of the convolution algebra of a groupoid are sections of a density bundle $\Omega ^{1/2}$ rather ...
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### Name for semigroupoid-like structure applicable to a water flow network in graph theory

Hope you are well. I am studying flow networks (flows of water) in graph theory. By flow network I mean a graph $G = (V,E)$ where $V$ is a set of $ℝ_{>0}$ labelled vertices (water tanks filled with ...
0answers
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### Group isomorphism between identity morphisms of fundamental groupoid and fundamental group of topological space

For any topological space $X$, I have defined $C=\Pi_1(X)$ by objects $\text{Ob}(C)=X$ and morphisms $\text{Hom}_C(x,y)=\text{HPath}(x,y)$, where $\text{HPath}(x,y)$ denotes the set of homotopy ...
2answers
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### Homotopy cardinality of weak quotients

Let $G$ be a group (regarded as a one-object groupoid) acting on a groupoid $X$ (i.e.: a functor $F: G \to \mathsf{Groupoids}$ sending the unique object of $G$ to $X$). Denote with $X//G$ the “weak ...
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### Definition of Haar systems, especially for groupoids $C^*$-algebra

In A Groupoid approach to $C^*$-algebras Jean Renault introduces left Haar systems. Say $G$ is a groupoid. Let $\Lambda=\{\mu^u,\,\,:\,\,u\in G^0\}$ be a family of Haar measures on $G$. Renault ...
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### Help with understanding the definition of a Hodge groupoid

I am reading this PhD thesis and I can't understand definition 6.21: (Hodge groupoid). $(X/S)_{Hod}$ is a groupoid whose object object is $X\times \Bbb A^1_S$ and whose morphism object $N_{Hod}$ is ...
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### How units of a Lie groupoid act on a manifold?

The definition of a Lie groupoid action given in Sébastien Racanière's notes (up to notation) says that the action of a Lie groupoid $\mathcal{G} \rightrightarrows M$ on a smooth manifold $Q$ consists ...
1answer
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### Cancellation law in a Lie groupoid

I'm playing around with the definition of a Lie groupoid following Eckhard Meinrenken's notes. I read the thing for the first time in my life like an hour ago, so assume that I don't know anything ...
3answers
191 views

### Simplicial sets which are not Kan complexes

A Kan complex is a simplicial set satisfying the horn-filler condition. What examples are there of simplicial sets which do not satisfy the horn-filler condition? In particular, what simplicial sets ...
1answer
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### Every normal subgroupoid is the kernel of a groupoid morphism

Sorry for the block of text from Kirill Mackenzie's chapter on groupoids. I have proven that $\natural,\natural_0$ is a groupoid morphism and that this quotient construction satisfies the groupoid ...
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### Why Isn't a piecewise-injective groupoid morphism injective?

The following is a question I had while reading Kirill Mackenzie's first book (RIP) on groupoids. He says that if a groupoid morphism is base-injective and piecewise-injective, then it is injective. I ...
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### Exact sequences of Groupoids?

My question is basically: Is there a good notion of complexes of groupoids and there exactness? I haven't found a definition online.
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### Bijection Between Maps of Covering Spaces and Maps of Fundamental Groupoid Covers

I'm reading May's Concise Course in Algebraic Topology. He states: My question is about this last corollary. How does the bijection $\text{Cov}(E, E') \to \text{Cov}(\Pi(E), \Pi(E'))$ immediately ...
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### Exemples of applications of “groupoidification” to linear algebra

I just read Baez's very nice blog notes about groupoidification, and around the beginning, he states : "From all this, you should begin to vaguely see that starting from any sort of incidence ...
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### Is the 2-Category of Groupoids Locally Presentable?

I am wondering if the 2-Category of groupoids is Locally Presentable? Locally presentable means the category is accessible and co-complete. Edit: It has been pointed out that the category of ...
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### Why are categories not called monoids (and why are monoids not called… mons?) [closed]

If groupoids are "indexed groups", wouldn't that same naming scheme imply that categories should be called "monoidoids", or more sensibly, why aren't categories called "monoids" and monoids called... "...
1answer
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### What's the correct notion of equivalence in a double category?

What's the appropriate notion of equivalence between two objects in a double category? At first I thought the answer was just an equivalence in one of the associated $2$-categories, but then I ...
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### Equivalence of Lie groupoids $\phi: H \rightarrow G$ induces an equivalence of categories $\phi^*: G\text{-spaces} \rightarrow H\text{-spaces}$.

In Orbifolds as Groupoids there is the notion of an equivalence $\phi: H \rightarrow G$ between Lie groupoids (2.4) and of $G$-spaces (5.1). Given a smooth functor $\phi: H \rightarrow G$ we can ...
1answer
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### Equivalence between the Dwyer-Kan loop groupoid and the fundamental groupoid

Let $X$ be a homotopy 1-type (a space with vanishing homotopy groups above degree one). It is a classical fact that $X$ can be recovered completely from its fundamental groupoid. On the the other ...
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### Inversion on an internal groupoid

Let $\mathcal{C}$ be a category with pullbacks, with $\mathscr{G}=({\bf Ob}_\mathscr{G},{\bf Hom}_\mathscr{G},cod,dom,{\bf 1}, \circ_{\mathscr{G}},-^{-1})$ an internal groupoid in $\mathcal{C}$. I'm ...